1. Field of the Invention
The present invention relates to a gas filter module which is suitable for filtration of a dry gas used in, for example, a production process of a semiconductor device, and a method of producing such a gas filter module.
2. Description of Related Art
Recently, as the performance and capacity of a semiconductor device are advancing, miniaturization and thinning of a device or a pattern are further expanded. According to advancement of miniaturization of a pattern and the like, also in various gas flows used in a production process, it is requested to remove fine particles (impurities) for the gas flows as much as possible.
In order to meet such a request, for example, JP-A-3-288504 (Unexamined Japanese Patent Publication (Kokai)) discloses a gas filter module having the following configuration. The gas filter module comprises a metal case having a gas inlet port and a gas outlet port, and a cylindrical (tubular) gas filter which is disposed in a gas flow path in the metal case. A shield member (made of, for example, a fluororesin) is disposed in a gap between the metal case and the gas filter. The gas filter is made of a porous member of ceramics such as alumina, and is disposed in the metal case so as to block the gas flow path. The shield member is fixed to the gas filter and the metal case so that a gas which once flows into the case via the inlet port cannot flow out via the outlet port unless it passes through a wall portion of the gas filter.
In the case where the shield member is made of an organic material such as a fluororesin, there is a fear that when the gas filter is subjected to a baking process (600.degree. C.) before it is used, for example, the organic material is decomposed to release a hydrocarbon gas, water vapor, and the like and such gasses are supplied as an impurity gas into the metal case. Furthermore, an organic material easily occludes various materials, and hence there is a further fear that occluded gasses are released as an impurity gas into the supply gas flow.
In order to comply with the problems which may arise in a practical use, a technique that the material of the shield member is changed to ceramics or a metal has been attempted. Specifically, a configuration is proposed in which the shield member is made of ceramics or a metal and the shield member is fixedly supported on a tubular gas filter element by a glass material or a metal solder material (JP-A 62-129104 and JP-A 2-172511).
In such a configuration, an impurity gas is prevented from flowing into the supply gas, and thermal deformation or the like is prevented from occurring. In this configuration, the ceramics shield member is so dense (gas impermeable) that a gas cannot pass through the member. In a gas filter (a plastics-free ceramics gas filter) or a gas filter module (a ceramics gas filter module of plastic free) in which the shield member is made of ceramics, it is possible to avoid or solve the problem in that, during a process where a gas is passed (or filtered) through the tubular gas filter element, the gas is contaminated with an impurity gas. However, such a gas filter or a gas filter module has the following disadvantage. When the tubular gas filter element made of ceramics and the shield member are integrally fixed to each other by a glass bonding agent (of the silica type or glaze), the assembling operation can be simplified. On the other hand, when a fluoric gas such as ClF.sub.3 is filtered and cleaned, the glass bonding agent is easily eroded. Therefore, the kind of a gas which can be filtered and cleaned is restricted, thereby producing a problem in that such a filter or filter module lacks versatility.
For example, the erosion resistance was checked by using a fluorine plasma under the following conditions (microwave output power: 560 W, carbon tetrafluoride: 155 sccm., oxygen: 75 sccm., exposure time: 20 min.times.5 times, and pressure: 0.17 torr). In a porous alumina member of a purity of 99.5%, the weight reduction rate was 0%. By contrast, in a porous alumina member (purity: 92%) joined (adhered) by aluminosilicate glass, the weight reduction rate was 1.7 to 1.9%.
A gas filter module in which the shield member is made of a dense metal or ceramics and the member is fixed to a ceramics filter and a metal case is proposed (JP-A HEI3-288504). As a specific method of fixing the metal case and the shield member, disclosed is a method in which, when two bottomed cylinders constituting the metal case are to be welded to each other, the cylinders are welded while clamping the shield member between the cylinders, whereby the metal case is formed and the metal case and the shield member are fixed to each other. In this case, when the clamping and joining portion of each of the two bottomed cylinders and the shield member does not have a high flatness, however, a gas leak occurs and the filtering function cannot be sufficiently performed. When a highly corrosive gas such as ClF.sub.3 for cleaning the interior of a semiconductor producing apparatus is passed through such a filter or a filter module, there arises a problem in that, particularly, the welded portion of the metal case is corroded. In order to prevent this problem from occurring, a sophisticated process technique is required. Therefore, such a filter or a filter module fails to have sufficient practicality. When such a filter is to be used for producing a semiconductor device, H.sub.2 O and O.sub.2 adsorbed by the filter are removed away prior, to the use by baking the whole of the filter including the case at a high temperature of about 600.degree. C. Also in the case where the gas to be used is changed, a similar baking process is conducted. In this way, particularly, a filter for producing a semiconductor device repeatedly undergoes a temperature change from room temperature to 600.degree. C. Particularly in the case where dense ceramics is used as the shield member, even when a flat face of a high accuracy is attained in an initial stage, a gap is gradually formed because the coefficient of thermal expansion of the ceramic material is largely different from that of the metal material constituting the metal case, thereby producing a problem in that a gas leak occurs.
Conventionally, a ceramics filter which is used as a gas filter has a structure shown in FIG. 13.
Referring to FIG. 13, 1 denotes a housing made of a metal such as stainless steel. The housing 1 consists of a housing rear portion 11, and a housing front portion 12, and is assembled by welding the portions together at a welding area 2 which is in a substantially middle area. A gas inlet port 3 for a gas to be filtered, and a gas outlet port 4 for a filtered gas are formed in the housing rear and front portions 11 and 12 of the housing 1, respectively. An inner space 5 is formed inside the center portion of the housing 1. A filter element 6 is disposed in the space so as to form a gap 8 between the element and the inner wall 7 of the housing 1. The filter element 6 is configured by a porous alumina member, and has a tubular shape as illustrated. The element is clamped at the longitudinal front and rear ends by packings 9.sub.1 and 9.sub.2 made of a synthetic resin such as Teflon. A press plate 10 is disposed outside the synthetic resin packing 9.sub.1 on the side of the housing rear portion 11 of the filter element 6, so as to grasp the packing 9.sub.1. A gas passage 1.sub.1 through which a gas can pass is opened in the press plate 10 so that an inlet gas is guided to the outside of the filter element 6.
When a gas is to be filtered by using the ceramics filter, the gas to be filtered flows into from a gas inlet pipe (not shown) connected to the gas inlet port 3 of the ceramics filter, though the gas inlet port 3. The gas passes through the gas passage 1.sub.1 opened in the press plate 10 and then reaches the gap 8 outside the filter element 6. The gas which has entered the gap 8 then passes through the interior of the filter element 6 as indicated by the arrows, whereby the gas is filtered and guided to the center portion of the filter element 6. Thereafter, the gas is sent from the center portion of the front end of the filter element 6 toward the housing front portion 12, and then discharged from the gas outlet port 4.
In the thus configured ceramics filter of the prior art, a synthetic resin such as Teflon is used as the packings serving as the shield member. Therefore, the filter has problems in that a gas is released in a high vacuum condition, and that the synthetic resin is deteriorated by the baking process or the like, hereby limiting the use of the filter. It has been requested to improve a filter.